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Old 12-01-2009, 12:05 AM   #1
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some electrical math need help


Ok I wanted to see what my load was with the A/C, oven, and dryer
on all at the same time.
I came across this site http://michaelbluejay.com/electricity/measure.html

After doing the bottom one for the analog meter I ended up with
one full rotation in 2 seconds after the math I ended up with 12,960.

Does this look right? Is it saying that its 13Kwh? And looking at the static number they have "3600" where did that number come from?

Thanks in advance.

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Old 12-01-2009, 04:56 AM   #2
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some electrical math need help


Ok after looking at more web pages im guessing the 3600 is in seconds

So with said appliances running at the same time plus other things
I'm looking at about an average 54 amps @ 240v at any one given time?

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Old 12-01-2009, 08:59 AM   #3
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For the large items (oven, etc.) you named, the numbers look appropriate.

At that moment the three items (and perhaps some lights or TV or other things also turned on) were together drawing about 54 amps at 240 volts or 13 kilowatts.

If this load went on, constant, for an hour you will have consumed 13 kilowatt hours.

If there were some 120 volt items turned on, chances are the current on one side of the 240 volt service won't be exactly the current on the other side, for example you might have 55 amps at 120 volts on one side and 53 amps at 120 volts on the other side, still a total of around 13 kw. (You could also say the latter as 53 volts at 240 volts across both sides plus 2 amps at 120 volts across one side.)
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Old 12-01-2009, 12:05 PM   #4
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http://www.wholesalesolar.com/pdf.fo...ower-table.pdf
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Old 12-01-2009, 07:27 PM   #5
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some electrical math need help


Thanks for the help,
So if I was to get a backup gen what this tells me is I need at least
54 amp one to run what was running when I took the measurement
correct?
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Old 12-01-2009, 07:50 PM   #6
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some electrical math need help


I'd never run the stove & dryer at the same time with power out
Who dries clothes while the power is out ?
And if the power was out I'd use fans instead of an AC
Depends upon how you connect the Gen & local codes
Some require an automatic gen to be able to handle the whole house load
The more amps you need, the higher the cost

Extended outage:
You'd cook food, run the AC then while you eat
Turn AC off, Do wash & dry - then turn AC back on

I have a 6500w unit = ~28a @ 240v
If I lose power all I really want to do is power my fridge, freezer, few lights & my pump for my hot water heating system

You'd need a 13 kw unit at a bare Min - pretty big & expensive
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Old 12-01-2009, 08:36 PM   #7
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some electrical math need help


All the generators I've seen are rated in watts. (Or kW for thousand watts.)

Here is a ballpark page to add up all the watts of everything you would want to have on at the same time. You can also look at the labels (nameplate) in what you have and correct the listed wattage with what you really have...
http://www.consumerreports.org/cro/r...lclulator.html

For example I have a 1000 watt microwave.

My refrigerator has a label inside which says 8.5 amps. So I can go to the following link and convert that to watts with the calculator under "single phase" and use 120 for the voltage...
http://www.jobsite-generators.com/po...lculators.html

Then the next link discusses "start-up wattage" of things with electric motors in them and the reason to get a larger rated generator than the wattage listed on the appliances...
http://www.lowes.com/lowes/lkn?actio...Generator.html

Some generators are "electronics friendly" and produce "clean" electricity...

First: Poor quality electricity from generators, etc...
http://www.jkovach.net/projects/powerquality/

Next: "Electronics Friendly" generator search...
http://www.google.com/search?hl=en&s...h&aq=f&oq=&aqi=
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Old 12-01-2009, 08:57 PM   #8
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Also new "portable" generators come with a GFCI outlet and this is not compatible with home electric wiring when a transfer switch is installed.

You would need a special transfer switch to also switch the "neutral".

The following product bulletin from a generator manufacturer recommends modifying the grounding on the generator to solve this problem...
(But the generator would no longer be OSHA compliant.)
http://www.gen-tran.com/assets/pdfs/...Bulletin20.pdf

Neutral switching transfer switch example...
http://www.briggsandstratton.com/pdf...m-Domestic.pdf
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Old 12-01-2009, 11:38 PM   #9
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some electrical math need help


The nameplate ratings on these appliances need to be known to come up with an accurate load calculation, especially for that A/C condensing unit because that's an inductive load and draws more power at start up than it does once it gets going.
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Old 12-02-2009, 03:48 PM   #10
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Thank you all for the help least now it gets me on the right path.

Much appreciated.

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